1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-165356, filed on Jun. 14, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a mobile communications system and, more particularly, to a method and system for transmitting data at the time of handover.
2. Description of the Related Art
In a mobile communications system or the like employing packet transmission protocols, for example, high-speed packet transmission protocols such as HSDPA (High Speed Downlink Packet Access) and EUDCH (Enhanced Uplink Dedicated Channel)/HSUPA (High Speed Uplink Packet Access), the prevention of data loss and service interruption at the time of handover (hereinafter, abbreviated as “HO” where appropriate) has become more and more important. In particular, as the automatic repeat request (ARQ) function of the radio link control (RLC) layer is shifted from a radio network controller (RNC) to a base station (Node B), it is a significant problem to be solved how lossless handover is effectively realized in the uplink data transmission from a mobile station to a base station.
One of the basic handover procedures in the uplink data transmission is described in 3GPP TR 25.813 V0.9.2 (2006-05), 9.1.7 (Network aspects) (3rd Generation Partnership Project; Technical Specification Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio interface protocol aspects (Release7)). The outline of a handover between base stations based on this procedure is as follows. A handover source base station (source eNB) forwards uplink packets that have been successfully (or completely) received from a user equipment/mobile station (UE), to a gateway (GW), discards packets that have not been completely received, and reports reception status information to the user equipment. During this period of time, if the user equipment's data destination is changed to the cell of a handover target base station (target eNB), the user equipment retransmits to the target base station the uplink packets that have been reported as being incompletely received. The target base station then forwards the completely received uplink packets to the gateway.
In this manner, even if a handover between the base stations occurs while uplink data is being transmitted, all the uplink data packets transmitted from the user equipment are forwarded to the gateway through any one of the source base station and target base station. According to this procedure, the user equipment can forward all uplink packets to the gateway, without transferring uplink packet and information such as an ARQ context from the source base station to the target base station.
However, in the case where the transmission of packets from a base station to a gateway is performed in order of the sequence numbers of the packets, when a packet is incompletely received, the packets with the sequence numbers subsequent to this incompletely received packet (even if they are completely received) are held by the base station. Then, after the packet corresponding to the incompletely received packet is completely received from a mobile station through the ARQ procedure, this completely received packet and the subsequent completely received packets are transmitted to the gateway.
Therefore, if a handover between base stations occurs in a state where one of the base stations holds untransmitted uplink packets, it is a significant matter how to handle these untransmitted uplink packets. A conceivable solving means is a method in which the source base station transfers these untransmitted uplink packets and ARQ context to the target base station, and when the target base station has completely received a retransmitted packet from the mobile station, the target base station transmits the packets including the retransmitted packet and subsequent uplink packets to the gateway.
However, according to the above-mentioned method, even when synchronization has been established between the mobile station and the target base station, the mobile station is left in a state of substantially no data transmission until the source base station transfers all the untransmitted uplink packets to the target base station and the target base station completely receives the retransmitted packet and then starts to transmit the uplink packets. During this period of time, the communication stays interrupted.
Moreover, the narrower the bandwidth of an interface between the base stations, the longer time it takes to transfer the packets between the base stations, and hence the longer duration of a communication interruption. The duration of a communication interruption is a factor directly related to the quality of radio service, particularly greatly affecting the user's feeling about usability. To reduce this duration of a communication interruption, the data transfer between the base stations needs to be carried out at as high speed as possible. However, it is undesirable to increase the transfer rate by widening the bandwidth of the interface between the base stations only for this purpose, also from the viewpoint of the effective use of network resources. Radio carriers may also be burdened with higher costs.
Furthermore, according to the method in which the untransmitted uplink packets and ARQ context are transferred from the source base station to the target base station upon the occurrence of a handover (hereinafter, this method will be referred to as forward transfer method), the ARQ process with the source base station is simply cleared when a handover occurs. Therefore, although this method can be applied to, for example, a system architecture in which the source base station and target base station are both physically connected to a gateway, it is difficult to flexibly adapt this method to other system architectures.